The invention relates to an apparatus for keeping at a determined temperature, for a determined time, a product mixture flowing therethrough and consisting of a liquid of low to medium viscosity containing solid pieces, which comprises a cylindrical vessel provided at one end with an inlet and at the other end with an outlet for the mixture, a conveyor mechanism acting selectively on the solid pieces in the mixture being disposed in the vessel, whereby the speed at which the solid pieces are conveyed through the vessel can be adjusted irrespective of the rate of flow of the liquid through the vessel, said conveyor mechanism having a rotor in the form of a shaft coaxially rotatable in the vessel and provided with at least one series of rotor arms mounted on the shaft, extending radially and lying substantially next to one another and spaced apart in the axial direction, the ends of said arms reaching to a point close to the inner wall of the vessel, and also having a stator in the form of at least one series of stator arms extending radially inwards from the wall of the vessel and lying substantially next to one another and spaced apart in the axial direction, said stator arms extending between the rotor arms and reaching to a point close to the rotor shaft.
In the "through flow" heating or cooling by a heat exchanger (for example a tubular heat exchanger) of liquids of low to medium viscosity contain a certain volume percentage of solid pieces (for example soup containing pieces of vegetables and meat), heat transfer takes place from the wall of the heat exchanger through the conveyor liquid, the flow of which may or may not be turbulent, to the outside of the solid pieces and thence to the core of these pieces. With this kind of heat transfer the conveyor liquid will have reached the desired end temperature more quickly than the core of the solid pieces entrained by the liquid. To enable the core of the solid pieces also to attain the same temperature as the liquid, the mixture is passed through the above-mentioned apparatus and the solid pieces are retained for determined time by a conveyor mechanism disposed in the vessel, while the liquid can flow through unhindered. The time during which the solid pieces are retained is so selected that the core of the solid pieces being heated or cooled by the liquid flowing over them will also have reached the required temperature. It may then occur inter alia that during the heating of the mixture the liquid will remain too long at the desired end temperature, so that the quality of the liquid is impaired.
In an apparatus known from Netherlands Patent Specification No. 8601221 the rotor arms are mounted in the form of a screw on the rotor shaft. The stator arms lie one behind the other, viewed in the axial direction. The mutual distance between the rotor arms and the stator arms is the same over the entire length of the apparatus.
The known apparatus has the shortcoming that it can retain only solid pieces whose measurements are above a certain value. If the mixture contains solid pieces of different dimensions, each requiring a different residence time in an apparatus of this kind, a plurality of the above described apparatuses, disposed serially, will be needed so that the cost of an installation equipped with these apparatuses is increased.